Computational fluid dynamic modelling of baffled open volumetric receiver operation.

Abstract

An Open Volumetric Receiver (OVR) is a type of solar energy receiver that is able to heat atmospheric air volumetrically via a porous absorber exposed to concentrated solar radiation, through which the air flows. OVRs have the potential to attain higher operational efficiency than tubular or cavity type receivers, and they have been extensively investigated for use in concentrating solar power (CSP) plants. In CSP applications, the hot air leaving the OVR is typically passed through a heat recovery steam generator to generate steam for the plant’s steam turbine, after which it is returned to the OVR. Here, it is injected back into the atmosphere near the receiver inlet where some of the warm return air is re-entrained along with fresh air entering it. The amount of air that is re-entrained into the OVR is quantified by the air return ratio, and the higher this ratio, the lower the energy lost from the receiver. One of the factors limiting the operational efficiency of OVRs is fairly poor ARR performance, in the region of 50 % for state-of-the-art OVR designs. This research aims to evaluate the effectiveness of the addition of the vertical air flow baffles in improving the air re-entrained performance of an OVR. The evaluation was carried out numerically using Ansys Fluent Computational Fluid Dynamics (CFD) modelling software. Prior to the core investigation, cold and hot flow validation studies were conducted with respect to a generalized porous absorber and an arrangement of HiTRec-II OVR modules. The corresponding CFD models were successfully validated against experimental data and the methodology used to model the HiTRec-II modules was used to model an arrangement of SolAir OVR modules and modified arrangements incorporating air flow baffles of varying lengths. OVR air re-entrainment performance was evaluated in terms of the module air outlet temperature. The performance of the SolAir modules was evaluated when exposed to wind at varying magnitude and direction. The results from this study were used as a baseline against which the performance predicted for the SolAir modules modified with baffles (of different lengths) could be compared. A comparison of the results indicates that there is a clear increase in mean module air outlet temperature, when air flow baffles are incorporated with the lowest being 2.5 % and highest being 60.7 % increase in the temperature among the wind conditions and baffle lengths investigated for the study. The increase in the temperature also implies an improvement in air re-entrainment and thus OVR efficiency. The results also suggested the existence of an optimal baffle length for the receiver modules, beyond which the air outlet temperature drops and the OVR efficiency deteriorates.